Ferromagnetic nanorings, mediums embodying same including devices and methods related thereto

ABSTRACT

Featured is a magnetic ring structure including at least a vortex magnetic state such as symmetrically and asymmetrically shaped nanorings ( FIG. 7C ), having small diameters (e.g., on the order of 100 nm). In particular embodiments, the width and thickness of the maxima and minima thereof are located on opposite sides of the nanoring. Also featured are methods for fabricating such symmetrically and asymmetrically shaped nanorings ( FIG. 1 ). Also featured are methods for controlling the reversal process so as to thereby create vortex states in such asymmetric nanorings by controlling the field angle ( FIG. 9 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. Ser. No. 11/885,846filed Dec. 8, 2008, now U.S. Pat. No. 7,983,074, granted Jul. 19, 2011,which is the national stage of International ApplicationPCT/US2006/09312 filed Mar. 14, 2006, which in turn claims priority fromU.S. Provisional Application No. 60/661,569 filed Mar. 14, 2005, thedisclosures/teachings of all of which are herein incorporated byreference.

STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH

The present invention was supported by grants from the National ScienceFoundation, grant number DMR00-80031. The U.S. Government may havecertain rights to the present invention.

FIELD OF INVENTION

The present invention relates to magnetic memory and more particularlyto ring-shaped memory elements, more specifically to asymmetricring-shaped memory elements and yet more specifically to asymmetricring-shaped memory elements having a vortex magnetic state.

BACKGROUND OF THE INVENTION

There are a number of different types of devices and methods for storageof data or information that do not lose the data or information thereon.Such storage devices or mediums are sometimes referred to asnon-volatile types of memory or storage devices and include magneticstorage mediums (e.g., magnetic disc or hard drives, magnetic tapedrives, flash memory cards, EEPROMS) and magneto-optical storage mediums(e.g., CD, CDROM, CDRAM, DVD, DVD±R or DVD±RW). Conventional randomaccess memory (e.g., SRAM, DRAM) on the other hand loses any data orinformation contained in the memory when power is lost to the devicethat uses the memory. Thus, conventional computers and processingsystems typical include different types of memory systems that usedifferent types of memory so as to meet operational needs orrequirements (e.g., RAM for use with the CPU and a non-volatilememory-magnetic hard drive for long term data storage).

Notwithstanding the efficacy of conventional long term magnetic storagedevices, new types of patterned media for information storage have beenexplored to replace the continuous magnetic films used in currentmagnetic storage in order to achieve higher storage density. M. Helm, K.Ounadjela, J. P. Bucher, F. Rousseaux, D. Decanini, B. Bartenlian, C.;Chappert, Science 1996, 272, 1782; K. J. Kirk, J. N. Chapman, S.McVitie, P. R. Aitchison, C. D. W. Wilkinson, Appl. Phys. Lett. 1999,75, 3683; M. Herrmann, S. McVitie, J. N. Chapman, J. Appl. Phys. 2000,87, 2994; C. A. Ross, Annu. Rev. Mater. Res. 2001, 31, 203. In patternedmedia, a bit, either “0” or “1”, is stored in a single magnetic entity,rather than in a bit consisting of a number of grains as in continuousfilm. Such new types of patterned media for information storage includemagnetic dots and magnetic disks. Both small dots and discs with sizesless than about 200 nm, however, have been found to have poor magneticstability especially when the separations between the small entities aresmall.

Nanoscale magnetic entities play a prominent role in many devices inwhich the shape and dimension of the entities dictate their intricatemagnetization configurations and switching characteristics. M. Helm, K.Ounadjela, J. P. Bucher, F. Rousscaux, D. Decanini, B. Bartenlian, C.Chappert, Science 1996, 272, 1782; K. J. Kirk, J. N. Chapman, S.McVitie, P. R. Aitchison, C. D. W. Wilkinson, Appl. Phys. Lett. 1999,75, 3683; M. Herrmann, S. McVitie, J. N. Chapman, J. Appl. Phys. 2000,87, 2994; A. Ross, Annu. Rev. Mater. Res. 2001, 31, 203. Of particularinterest are small circular disks that can acquire the so-called vortexstate, in which the magnetic flux is confined within the magnetic entityand creates no stray fields, so that the cross-talk between entities canbe reduced. However, the vortex core (with magnetization pointing out ofthe plane of the disk) tends to destabilize the vortex state, whichconsequently is replaced by the single-domain state in sufficientlysmall disks. In supermalloy disks, the transition from vortex state tosingle-domain state occurs at a diameter of a few hundred nanometers.However, the instability of the vortex core in a small circular disk canbe circumvented by removing the central region of the disk to form aring structure, the so-called “nanorings”. Nanorings possess highlystable vortex states in which the magnetic moments form circular loopsalong the circumference. The chirality of the vortex states, clockwiseand counterclockwise, can be utilized to store information of “0” and“1”. For nanorings with 100 nm to 500 nm in diameter, there are twoprocesses for switching their magnetizations: vortex formation processand onion rotation process.

Experimental results for micron-diameter rings and 300-800 nm diameteroctagonal ring structures show the existence of just two differentmagnetic states: one being the flux-closure or vortex state and theother a bi-domain state with two 180 degree domain walls, called anonion state. There also has been reported in US2004/0211996 anothermetastable state called a twisted state for smaller diameter rings. Thistwisted state contains a 360 degree domain wall and can exist over awide range of applied fields. To attain such a twisted state, thenanoring is configured so as to include a deviation (e.g., notch),artifact so that the domain wall is pinned at the locations of thedeviations. A nanoring having such a deviation or configuration wasreferred to as being asymmetric. These nanorings also were fabricated bya liftoff process from ring-shaped patterns written into a resist layerby electron-beam lithography.

In such structures, each ring can store a bit of information dependingon its magnetic state. The rings are written by applying magnetic fields(the fields are produced by passing currents through adjacent conductivelines). The data-bit in the rings is read back by detecting the rings'electrical resistance, which depends on their magnetic states. Thedependence of resistance on magnetic state is called magnetoresistance.To use magnetoresistance for data readback it is most convenient to makethe memory element out of a magnetic multilayer, for instance twomagnetic layers separated by a non-magnetic spacer. In such a multilayer(called a spin-valve or tunnel junction), the resistance can vary by upto about 10-50% depending on the relative magnetization directions ofthe two magnetic layers and the structure of the multilayer.

In sum, small magnetic nanorings can not only maintain stable vortexstates, but also hold the potential for information storage in the twochiralities of the circulating magnetization. These properties ofnanorings have also led to the proposal of high-density, vertical,magnetic, random access memory (VMRAM) consisting of multilayerednanorings with exceptional stability and desirable switchingcharacteristics. J. G. Zhu, Y. Zheng, G. A. Prinz, J. Appl. Phys. 2000,87, 6668.

For these reasons, the study of magnetic nanorings of various sizes hasbeen actively pursued recently. Electron-beam lithography (EBL) is onemethod used for fabricating magnetic nanorings. Most of the magneticrings previously reported have been in the micrometer-size range, withfew reports existing on sub-micrometer-sized nanorings. The arrays ofmagnetic rings made by EBL usually have a small number of rings (e.g.,less than 10³) with low areal densities (e.g., 0.5 rings μm² or less).As such, the magnetic signal of the magnetic nanorings has been too weakfor full characterization using magnetometry. Instead, the magneticcharacteristics have been measured or inferred by surface magneto-opticKerr effect (MOKE) measurements, resistance measurements, Hall-sensormeasurements, and magnetic force microscopy. In these cases, theexternal applied magnetic field could only be applied in certaindirections, and not all, so as not to interfere with the specificmeasuring technique used. Also, magnetic nanorings fabricated by e-beamlithography are limited to a small area with low areal density.

The present inventors have discovered from magnetic measurements ofsymmetrically shaped nanorings, that when the diameters of symmetricallyshaped magnetic nanorings are of the order of 100 nm, the magneticreversal undergoes two processes with similar probabilities. One is thevortex reversal process, in which the vortex states can be sustained.The other one is the rotating onion process that involves no vortexstates.

It thus would be desirable to provide a ferromagnetic nanoring includingan assemblage of such nanorings having small diameters and methods formaking such ferromagnetic nanorings. It would be particularly desirableto provide such methods for fabricating such ferromagnetic nanoringshaving controlled dimensions including diameter, width, and thickness aswell as areal density. It also would be desirable to provide suchmethods for fabricating nanorings that yield nanorings that aresymmetrically or asymmetrically shaped. It also would be desirable toprovide methods for controlling application of the magnetic field to thenanoring(s), in particular the asymmetrically shaped nanorings so as tothereby control the formation of a vortex state in such nanorings.

SUMMARY OF THE INVENTION

The present invention features symmetrically and asymmetrically shapednanorings, having small diameters (e.g., on the order of 100 nm) as wellas methods for fabricating such symmetrically and asymmetrically shapednanorings. Also featured are methods for creating vortex states in suchasymmetric nanorings by controlling the field angle.

According to one aspect of the present invention, there is provided amethod for fabricating a magnetic-ring structure having one of asymmetric or asymmetric shape on a substrate. The term asymmetric asused in the subject application shall be understood to mean a ringstructure whose width and thickness is controlled but which does notcreate a condition by which a domain wall is pinned at a given location.In particular embodiments, such methods are lithographic-less that isnot directly formed using electron beam lithography techniques.

More specifically, such methods include forming or creating a templatesuch as an array of nanospheres on the surface of a substrate andapplying a magnetic material to such a surface. To form symmetricallyshaped nanorings, the magnetic material is etched by applying theetching beam at an angle that is generally normal to the surface of thesubstrate so that the magnetic material remaining is that which isgenerally disposed beneath the nanosphere. To form asymmetrically shapednanorings, the substrate is tilted so as to be at an angle (an obliqueangle) with respect to the general direction of the etching beam. Inparticular embodiments, the substrate is tilted at an angle between 0and 90 degrees. As each nanosphere thus blocks the etching beam todifferent degrees, the ring of magnetic material formed along one partof the circumference will be generally characterized as having a largerwidth or cross-section than another part of the circumference. Followingthe formation of such nanorings, the method can include applying aprotective material and/or removing each nanosphere.

According to another aspect of the present invention, there is featureda method for recording information in such nanorings. More particularly,such methods include controlling the angle of the field with respect toan asymmetric axis of the nanoring so as to be within a predeterminedrange of angles so as to thereby control the formation of a vortex stateas compared to an onion state in the nanorings. More specifically thefield angle is controlled with respect to the asymmetric axis so thepercentage of vortex states in the asymmetric rings is in the range offrom 41% to 97%.

The symmetrically shaped and asymmetrically shaped nanorings of thepresent invention have a diameter (inner diameter) of about 50 nm andlarger.

The asymmetric nanorings of the present invention are configured orshaped so that the width of the asymmetric nanoring varies as a functionof radius or the locations on the circumference of the nanoring. In moreparticular embodiments, the width of the asymmetric nanoring variescontinuously about the circumference of the nanoring. In more particularembodiments, the width of the asymmetric nanoring varies so that theminimum width and the maximum width are spaced from each and so as to beon opposite sides of the nanoring, more specifically they are spacedabout 180 deg from each other. In yet more particular embodiments, themaximum width of a nanoring is in the range of between 2 and 5 timeslarger than the minimum width of the nanoring.

The asymmetric nanorings of the present invention also are configured orshaped so that the thickness of the asymmetric nanoring varies as afunction of radius. In more particular embodiments, the thickness of theasymmetric nanoring varies continuously about the circumference of thenanoring. In more particular embodiments, the thickness of theasymmetric nanoring varies so that the minimum thickness and the maximumthickness are spaced from each and so as to be on opposite sides of thenanoring, more specifically they are spaced about 180 deg from eachother. In yet more particular embodiments, the maximum thickness of ananoring is in the range of between 2 and 5 times larger than theminimum thickness of the nanoring.

Other aspects and embodiments of the invention are discussed below.

BRIEF DESCRIPTION OF THE DRAWING

For a fuller understanding of the nature and desired objects of thepresent invention, reference is made to the following detaileddescription taken in conjunction with the accompanying drawing figureswherein like reference character denote corresponding parts throughoutthe several views and wherein:

FIG. 1 is a high-level flow-diagram of the process for fabricating asymmetric or asymmetric nanoring according to the present invention;

FIG. 2 is an illustration depicting the formation of a monolayer ofnanospheres on the substrate;

FIG. 3 is an illustration depicting the formation of a layer of magneticmaterial on the substrate surface having the nanospheres;

FIG. 4A is an illustration depicting ion beam etching to formsymmetrically shaped nanorings;

FIG. 4B is an illustration depicting ion beam etching to formasymmetrically shaped nanorings;

FIGS. 5A,B are micrograph illustrations of top views of the substratesurface with the nanorings using scanning electron microscopy (SEM) thatshows the structure of symmetrically shaped nanorings (FIG. 5A) andasymmetrically shaped nanorings (FIG. 5B);

FIG. 6 is a top view SEM micrograph illustrating areal density ofasymmetric rings having a nominal 100 nm diameter;

FIGS. 7A,B are a close-up views of a micrographs of symmetric nanorings(FIG. 7A) and of asymmetric nanorings (FIG. 7B) having a 100 nm nominaldiameter;

FIG. 7C is a top view of an asymmetric nanoring to define the symmetryand asymmetry axes of an asymmetric nanoring;

FIGS. 8A,B are graphical views of hysteresis loops for asymmetricnanorings etched at an 14 deg. incident angle and a symmetric nanoringsetched at a normal angle;

FIG. 9 is a graphical view of field angle dependence of coercively andloop area;

FIGS. 10 A,B are graphical views of the vortex reversal process (FIG.10A) and the rotating onion process (FIG. 10B); and

FIG. 11 is a graphical view illustrating the relationship between vortexpercentage and field angle.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the various figures of the drawing wherein likereference characters refer to like parts, there is shown in FIG. 1 ahigh level flow diagram of the process for fabricating or formingnanorings that are symmetrically shaped or asymmetrically shaped. Theprocess is started, Step 100, by obtaining and arranging the materialsso as to carry out the below described process. For example, a siliconwafer is pre-coated with a 20 nm layer of sputtered titanium andoxidized in air (e.g., 300° C., 15 minutes) for use as a substrate inthe below described process.

After starting the process, a template, such as a template ofnanospheres, is formed on a surface of the substrate, Step 110. Inparticular embodiments, a conventional layer by layer coating method [P.Hanarp, D. S. Sutherland, J. Gold, B. Jasemo, Colloids and Surfaces A2003, 214, 23] is used to make the substrate surface positively charged.The three layers in sequence are: (1) poly(diallyldimethylammoniumchloride) (PDDA), (2) poly(sodium 3styrenesulfonate) (PSS), and (3)aluminum chloride hydroxide (ACH). The substrate is immersed in eachsolution for 30 seconds, rinsed by deionized (DI) water and then driedby blowing N₂. Thus, the substrate includes these three layers plus thetitanium layer.

The substrates are then immersed in a 0.125˜0.5 wt-% polystyrene (PS)sphere suspension for 5 minutes, finally rinsed by de-ionized water anddried by blowing N₂. The negatively charged polystyrene spheres areattached to the positively charge substrates due to electrostaticattraction. This process thereby creates a monolayer of polystyrenenanospheres on the surface of the substrate.

After creating the template, the nanorings are created using thenanospheres, Step 120. More particularly, the substrates having thenanospheres are mounted to a substrate or sample holder that is disposedwithin a sputtering system such as a magnetron sputtering system. In yetmore particular embodiments, the substrate or sample holder isconfigured so as to be tiltable as hereinafter described.

The sputtering system is thereafter operated so that a layer of magneticmaterial as is known to those skilled in the art is applied to thesurface of the substrate on which is disposed the nanospheres, Step 122.The magnetic material is any of a number of materials known to thoseskilled in the art that are appropriate for the intended use andparticular manufacturing method. In exemplary embodiments, a cobalt (Co)film is sputter-deposited from a 99.995% pure Co target in a magnetronsputtering system with 6×10⁻⁸ Torr base pressure and 6 mtorr Argon(Ar)-sputtering pressure. The sample or substrate holder(s) is/are movedso the substrate is swept across the sputtering plasma in order to haveuniform thicknesses everywhere. The sputtering rate of Co at 50 mAsputtering current is 1.78 Å/sec. As is known in the art, the filmthickness is controlled by the sputtering time.

Following the deposition of a layer of magnetic material (Step 122), themagnetic material layer is etched or processed using techniques known tothose skilled in the art to selectively remove certain of the magneticmaterial (e.g., the magnetic material other than that underneath thenanospheres) and depending upon whether the process is to yieldsymmetrically shaped or asymmetrically shaped nanorings, Step 124.

In an exemplary illustrative embodiments, a broad beam Argon (Ar⁺) ionsource is used to etch the samples. As indicated above, the ion beametches off all the magnetic/cobalt (Co) material except the Co materialunderneath the PS spheres. To obtain symmetrically shaped nanorings, theion beam is arranged so that the ion beam is generally normal to thesubstrate surface, Step 126. When so arranged the ion beam etches thesurface at a normal angle and symmetrically shaped rings are formedunder the nanospheres as illustrated in FIG. 4A.

If asymmetrically shaped nanorings are to be produced, thesample/substrate holder is tilted so that the substrate is tilted by anangle α and so the ion beam incident angle with respect to the substratesurface also will be α, Step 128. In this way, asymmetric nanorings areformed under the spheres as illustrated in FIG. 4B. In illustrativeembodiments, a Veeco 3 cm DC Ar⁺ ion source is used to perform theetching, where the working conditions are: 500 V beam voltage, 66 mAbeam current, 1000 V acceleration voltage and 0.23 mtorr Ar pressure.The etching rate of cobalt under these conditions is 10.54 Å/sec. Infurther embodiments, the substrate tilting angle α is adjusted from 0°to 20° continuously inside the chamber without breaking the vacuum. Itshould be noted that the SEM images and magnetic measurements ofasymmetric nanorings discussed below, unless otherwise indicated, arefor a substrate that was etched at a 14° incident angle.

After such etching, forming the nanorings (Step 120), a protectivematerial is deposited upon the etched surface to prevent oxidation ofthe magnetic nanorings, Step 130. Such a protective layer is of amaterial that does not affect the magnetic properties of the nanorings.In illustrative embodiments, the whole surface is coated with 3 nmprotective layer of gold (Au) or copper (Cu), which protection layer isdeposited by magnetron sputtering. In addition, following such etchingit is within the scope of the present invention to remove thepolystyrene nanospheres such as by chemically etching or chemicallydecomposing.

Symmetrically shaped and asymmetrically shaped nanorings were formedusing the above techniques. The following reports the structure andcharacterizes the properties of such nanorings.

A scanning electron microscope (SEM) was used to study the structure ofthe nanorings formed using the above described techniques, where FIGS.5A and 5B are the top view micrographs of symmetric and asymmetriccobalt (Co) nanorings, respectively. In the micrographs, the brightareas show the Co rings and the central areas are the exposed PSspheres.

From the micrographs, the areal density of symmetric nanorings in FIG.5A was determined to be about 45 rings/um², or 30 Giga-rings/in². Theareal density of the asymmetric nanorings shown in FIG. 5B appears low,however, this is due to the low concentration of polystyrene nanospheresin the solution in the preparation step.

In contrast to FIG. 5B, there is shown in FIG. 6 a micrograph that showsthat asymmetric nanorings can be formed on the substrate with a higherareal density. Although the nanorings are close to each other in FIG. 6,they remain separated from each other. The average density of nanoringsin this micrograph is about 35 rings/μm², or 23 Giga-rings/in².

There is shown in FIGS. 7A,B a high magnification top view SEM image ofthe symmetric nanorings (FIG. 7A) and a high magnification top view SEMimage of the asymmetric nanorings (FIG. 7B). From the image for theasymmetric nanorings, one can determine that the inner diameter is 100nm, the width of the left wall of the nanoring is 50 nm and the width ofthe right wall is 10 nm. The symmetry and asymmetry axes of anasymmetric nanoring are shown in FIG. 7C.

Referring now to FIGS. 8A, B there are shown graphical views of themeasured hysteresis loops of asymmetric and normal nanorings,respectively. The hysteresis loops were measured by a vibrating samplemagnetometer (VSM) with the magnetic field along various directions.

More specifically, in FIG. 8A, ten hysteresis loops with H applied atdifferent directions of the asymmetrical nanorings are plotted together.They share some similarities but with important differences. The similarcharacteristics are: (1) two switching fields: the first switchinghappens at a small field between 140 and 280 Oe, the second switching ata much larger field about 750 Oe; (2) squareness (defined as the ratioof remanence to saturation magnetization) is always 65%; (3) slowchanges of magnetization when the field value H is between the twoswitching fields. However, these loops also have important differences.When the field angle changes from 0° to 90°, the magnetization valuesbetween the two switching fields as well as the second value ofswitching field systematically decrease. In contrast, the hysteresisloops of the symmetric rings measured at various angles are essentiallythe same, as revealed in FIG. 8B.

The strong dependence of switching characteristics on the field angle ofthe asymmetrical nanoring is better illustrated in FIG. 9, where thevalues of coercivity and the hysteresis loop area are plotted versus thefield angle. Both parameters are periodic functions of field angle witha period of 180°, and have maxima (minima) when fields are along theasymmetry (symmetry) axis.

To understand why the field angle can affect the switching behaviors ofasymmetric nanorings but not the symmetric ones, one first needs to knowthe switching processes of symmetric nanorings. As indicated in F. Q.Zhu, D. L. Fan, X. C. Zhu, J. G. Zhu, R. C. Cammarata, C. L. Chien,Advanced Materials 2004, 16, 2155, such studies show that the magneticreversal of a symmetric magnetic ring can undergo two processes. One isthe vortex reversal process, during which the vortex states can besustained when field value is between the two switching fields. Thecharacteristic of its hysteresis loops is the very small magnetizationsbetween the two switching fields, as shown in FIG. 10A. The otherprocess is the rotating onion process that involves no vortex states andhas only one switching field. In this case, the magnetization valuereaches 65% of the saturation value when magnetic field is beyond thecoercivity and then gradually increases to saturation value with themagnetic field. The two processes can occur simultaneously in the samesample with different probabilities. The overall hysteresis loop istherefore the weighted superposition of these two processes.

The actual percentage of the vortex reversal process can be calculatedby numerically fitting the experimental data with the two simulatedloops in FIGS. 10A,B. For uniform nanorings of 100 nm in diameter, asshown in FIG. 8B, the vortex reversal process has a probability of 40%(this percentage does not depend on the field angle). For asymmetricnanorings etched at 14° incident angle, the hysteresis loops dependstrongly on the field angle. The calculated percentage (P) of vortexreversal process is plotted as a function of the field angle θ in FIG.11. The calculated percentage (P) depends on the field angle θ almostlinearly when 0 is larger than 30° with a slope of about 0.8%/degree. Inother words, every 1° increase of the field angle will increase thevortex process by about 0.8%. When the field is along the symmetry axis,P is 41%, however, when field is along the asymmetry axis, P is 97%.Thus, any value of P above 41% can be obtained by setting the magneticfield along an appropriate angle.

According to further aspects of the present invention, the ring widthand thickness for asymmetric nanorings vary along the circumference,with their minima and maxima separated by about 180° (i.e., the minimaand maxima are located on opposite sides or portions of the nanoring).The domain wall energy E(θ), therefore, changes with the angularposition θ of the domain wall as plotted in the inset of FIG. 11. If theinitial magnetic field is along the symmetry axis, one domain wall(DMW1) will be generated at the thinnest location A and the other domainwall (DMW2) at the thickest location A′ after the field is removed (FIG.8A). Under a reversal magnetic field, DMW1 and DMW2 still have twopossible directions to move, and the situation is not very differentfrom that of the symmetric rings. The vortex probability P is close tothat of a uniform nanoring. If the initial field is along the asymmetryaxis, however, DMW1 and DMW2 will be generated at the middle locations Band B′ with the largest slope. Both domain walls tend to slide to thesame energy minimum position A′. As a consequence, the vortex formationprocess is enhanced. The higher the asymmetry and the larger thegeometrical slope, the higher P is, as shown in FIG. 11.

Others have imposed some form of additional anisotropy into the ringsand disks to break the circular symmetry; these include creating twonotches at the opposite ends in the rings as local domain wall pinningcenters and fabricating elliptical rings. These schemes, however,introduce anisotropy only in one axis. Furthermore, the large sizes(micrometers) of the rings also deprive the observation of the onionrotation process. In contrast, for the asymmetrical nanorings of thepresent invention, the much smaller size (on order of 100 nm) of thenanorings allows the observation of the onion rotation process. Moreimportantly, the asymmetric nanorings of the present invention create anon-local and continuously varying anisotropy by changing the nanoringcross section along the circumference. As a consequence and as describedherein, the fractions of the vortex and the rotating-onion processes canbe controlled by the direction of the applied magnetic field.

As described herein, for the patterned media of the present invention,each magnetic entity or nanoring can be used to store a bit “0” or “1”.Further such nanorings, in particular asymmetrically shapedferromagnetic nanorings can be utilized in magnetic storage devices inwhich each nanoring represents one bit. In addition, such asymmetricallyshaped ferromagnetic nanorings can be used in vertical magnetic randomaccess memories (VMRAM), in which each bit is a stack of circular ringssuch as that described in J. G. Zhu, Y. Zheng, G. A. Prinz, J. Appl.Phys. 2000, 87, 6668.

Although a preferred embodiment of the invention has been describedusing specific terms, such description is for illustrative purposesonly, and it is to be understood that changes and variations may be madewithout departing from the spirit or scope of the following claims.

Incorporation by Reference

All patents, published patent applications and other referencesdisclosed herein are hereby expressly incorporated by reference in theirentireties by reference.

Equivalents

Those skilled in the art will recognize, or be able to ascertain usingno more than routine experimentation, many equivalents of the specificembodiments of the invention described herein. Such equivalents areintended to be encompassed by the following claims.

1. A method for fabricating a magnetic ring structure, comprising thesteps of: creating a template providing one or more artifacts that areconfigured so as to create a nano-sized ring shape on/in a surface of asubstrate; forming a layer of magnetic material over the substratesurface and the one or more nano-sized artifacts; and selectivelyremoving a part of the magnetic material layer in such a manner so as toleave a nano-sized ring shaped member on/in the substrate surface. 2.The method of claim 1, wherein: the one or more artifacts are nanosphereshaped members, which members are affixed to the substrate surface; andsaid selectively removing comprises removing the magnetic material fromthe nanosphere shaped members except for the magnetic material disposedbeneath the nanosphere shaped member.
 3. The method of claim 2, whereinsaid selectively removing includes selectively etching portions of themagnetic material layer from the nanosphere shaped members so as toleave the magnetic material disposed beneath the nanosphere shapedmember.
 4. The method of claim 1, wherein said selectively removingincludes selectively etching portions of the magnetic material layerformed over the one or more nano-sized artifacts so as to leave themagnetic material forming the nano-sized ring shaped member.
 5. Themethod of claim 1, wherein said selectively removing includesasymmetrically selectively removing portions of the magnetic material sothat one of the width and thickness of each of the nano-sized ringshaped members remaining is asymmetric.
 6. The method of claim 2,wherein said selectively removing includes asymmetrically selectivelyremoving portions of the magnetic material so that one of the width andthickness of the magnetic material beneath and about each of thenanosphere shaped member is asymmetric.
 7. The method of claim 3,wherein said selectively etching includes asymmetrically selectivelyetching portions of the magnetic material so that one of the width andthickness of the magnetic material beneath and about each of thenanosphere shaped member is asymmetric.
 8. The method of claim 1,wherein: the one or more artifacts are nanosphere shaped members, whichmembers are affixed to the substrate surface; and said selectivelyremoving includes selectively etching parts of the magnetic materiallayer, wherein said selectively etching includes: etching the magneticmaterial with an ion beam; tilting the substrate to an angle a so theion beam is at an incident angle of other than normal to the substratesurface having the magnetic material layer therein.
 9. The method ofclaim 8, wherein αis defined by the relationship; 0<α≦20 deg.
 10. Themethod of claim 5, wherein both of the width and thickness areasymmetric.
 11. A method for fabricating a magnetic ring structure,comprising the steps of: creating a template providing one or moreartifacts that are configured so as to create a nano-sized ring shapeon/in a surface of a substrate; forming a layer of magnetic materialover the substrate surface and each of the one or more nano-sizedartifacts; selectively etching a part of the magnetic material layer soas to leave the nano-sized ring shaped member; the one or more artifactsare nanosphere shaped members, which members are affixed to thesubstrate surface; and wherein said selectively etching includes:etching the magnetic material with an ion beam; and tilting thesubstrate to an angle αso the ion beam is at an incident angle of otherthan normal to the substrate surface having the magnetic material layertherein.
 12. The method of claim 8, wherein αis defined by therelationship; 0<α≦20 deg.
 13. The method of claim 2, wherein saidforming includes forming the layer of magnetic material over an exposedsurface of each of the nanosphere shaped members.
 14. The method ofclaim 1, wherein: the template provides a plurality of artifacts;wherein said forming further includes forming the layer of magneticmaterial over each of the plurality of nano-sized artifacts; and whereinsaid selectively removing comprises selectively removing the magneticmaterial layer formed over each of the plurality of nano-sized artifactsin such a manner so as to leave a plurality of nano-sized ring shapedmembers.
 15. The method of claim 14, wherein: each of the plurality ofartifacts comprise a nanosphere shaped member, each nanosphere beingaffixed to the substrate surface; and said selectively removingcomprises selectively removing the magnetic material layer from each ofthe plurality of nanosphere shaped members except for the magneticmaterial that is disposed beneath each nanosphere shaped member.
 16. Themethod of claim 1, wherein said selectively removing comprisesselectively removing the magnetic material layer that is formed over theone or more nano-sized artifacts, thereby leaving the nano-sized ringshaped member for each of the one or more nano-sized artifacts.
 17. Themethod of claim 1, wherein said selectively removing comprisesselectively removing the magnetic material layer formed over each of theone or more artifacts and a part of the magnetic material about each ofthe one or more artifacts so that one of the width and thickness of thenano-sized ring shaped member for each of the one or more nano-sizedartifacts is asymmetric.